1. Field of the Invention
The present invention relates generally to transimpedance amplifiers.
2. Description of the Related Art
Transimpedance amplifiers typically deliver a single-ended output voltage signal in response to a differential input current signal. For effective use in a variety of systems (e.g., cable communication systems and optical transmission systems), they must generally drive low-impedance loads and provide output signals with extremely low distortion (e.g., harmonic spectral lines reduced  less than 50 dB below the carrier).
In order to meet these requirements, the output circuit of an exemplary upstream cable modem amplifier comprises a differential pair of transistors and a 2:1 (voltage ratio) transformer. A resistor across the collectors of the differential pair provides a nominal output impedance of 300 ohms and the transformer is required to provide a single-ended output, to match this output to a 75 ohm load and to maintain an acceptable bandwidth. The output circuit of another exemplary transimpedance amplifier requires a complex feedback system that comprises a number of diodes, resistors, capacitors and transistors.
In contrast to these exemplary amplifiers, a preferred transimpedance amplifier should provide a low-distortion output voltage signal across low-impedance loads and be formed with simple, small, inexpensive and reliable structures that are compatible with integrated-circuit fabrication processes.
The present invention is directed to transimpedance amplifiers that generate low-distortion output voltage signals with simple, inexpensive structures that are compatible with integrated-circuit fabrication processes.
These amplifiers include a current processor and a complementary output stage. The processor provides in-phase upper and lower current signals Supr and Slwr in response to a differential input current signal Sdiff and alters amplitudes of these signals in response to a common-mode input current signal Scm. The complementary output stage has upper and lower transistors that provide the output voltage signal in respective response to the upper and lower current signals and with distortion that is reduced by the altered first and second amplitudes. The common-mode input current signal Scm can be set to a predetermined value.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.